This invention relates to catalytic combustors for wood-burning stoves. More particularly, this invention relates to a catalytic combustor assembly that can be mounted to an existing stove.
The use of wood stoves has boomed in recent years as the burning of wood has become a viable alternative to electricity, gas, and oil for heating homes. With increased wood burning in urban areas, however, has come a corresponding increase in air pollution, especially in unburnt hydrocarbons found in smoke. In some locations, the smoke from wood stoves is now of such major concern that legislation has been passed to limit smoke emissions. In Oregon, for example, recent state legislation applicable to new stoves imposes a limit on the grams of smoke per hour that a wood stove may emit.
Most conventional wood stoves cannot meet the statutory emission limitations imposed by such legislation because these stoves cannot heat the volatile gases comprising the smoke sufficiently to burn the gases completely. These products will burn completely only if raised to a much higher temperature. In response to this need, the technology has developed catalytic combustors that are instaled within wood-burning stoves at the time of manufacture. Typically, the combustor comprises a noble metal catalyst mounted on a ceramic substrate. The catalyst lowers the ignition temperature of the hydrocarbons and thus temperature needed in the stove. As the hydrocarbons burn, they raise the temperature of the catalyst and substrate, which correspondingly increases the catalytic activity. New stoves that are manufactured with catalytic combustors built integrally within the stove firebox are disclosed in U.S. Pat. No. 4,479,921 to Allaire et al. and U.S. Pat. No. 4,319,556 to Schwartz et al. The combustor is preferably positioned within the firebox to optimize the performance of the catalyst.
The drawback of designs such as disclosed in these patents, of course, is their sole application to newly manufactured stoves. They are not adaptable to existing stoves which number in the millions and are unlikely to need replacement for many years to come. Recognizing this drawback and the possibility of legislation mandatory retrofitting of existing stoves, add-on catalytic combustors have been developed for adding a catalyst to an existing stove. These add-on combustors take the form of a unit installed conveniently within the stove pipe of the stove beyond the flue outlet. Examples of such units include the devices disclosed in U.S. Pat. No. 4,550,668 to Piontkowski and U.S. Pat. No. 4,476,852 to Lee et al, the Uni-Com insert manufactured by Riteway Company and the VERSAGRID catalytic converter kit manufactured by Applied Ceramics, Inc., of Atlanta, Ga.
Although these insertable units provide a means for retrofitting existing stoves, the in-pipe design they utilize is inherently limited because of the distance of the catalyst from the center of combustion within the stove. First, the converter is not heated quickly when the stove is first fired. A significant amount of polluting smoke must then be diverted to bypass the catalyst until it is heated sufficiently to be effective. Secondly, a cooler catalyst does not burn the volatile gases sufficiently. Particulate matter within the gases can plate out on the catalyst and render it inoperative. Thirdly, a hotter and thus higher fuel-consuming fire is needed to heat the catalyst sufficiently.
In light of these shortcomings, a preferable design for an add-on catalytic combustor assembly is one that can be heated quickly and to a higher degree than existing combustors without requiring the building of a hotter fire.